Polar lipids, especially those found in milk, are composed of three major groups of lipids:                (i) Glycerophospholipids such as phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol (PI), and their derivatives.        (ii) Sphingoids or sphingolipids such as sphingomyelin (SM) and glycosphingolipids comprise of cerebrosides (neutral glycosphingolipids containing uncharged sugars) and the gangliosides (GG, acidic glycosphingolipids containing sialic acid) and their derivatives.        (iii) Cholesterol and its derivatives.        
Polar lipids are constituent components of some foods, although their quality and quantity vary considerably depending on the food source. Dairy products such as milk and eggs are the richest sources of these polar lipids; the glycerophospholipids are also present in plants such as soybean. However, a child may need to consume a large amount of bovine milk (˜2,000 ml) to obtain a reasonable amount of polar lipids. In addition, consumption of eggs is also limited in infants and children due to the possibility of the allergenic protein properties of eggs. Plants are not a source of some of the polar lipids and in particular gangliosides because they are unable to synthesize sialic acid, a component of gangliosides. Furthermore the quality of polar lipids in plants is totally different than milk polar lipids not only due to the fatty acid profile but also due to the ratio of the individual phospholipid components of these lipids.
Lactoferrin, an iron-binding glycoprotein, is one of the major multifunctional agents present in human milk. It has the capacity to bind two molecules of iron in a reversible fashion and can facilitate the uptake of iron within the intestines. Further, lactoferrin has been shown to be both bacteriostatic and bactericidal, and it aids in preventing intestinal infections in humans, especially in pediatric subjects. Additionally, human lactoferrin appears to have a positive effect on the symptoms of diarrheal diseases.
Moreover, the infant gut microflora is rapidly established in the first few weeks following birth, and it has a great impact on an infant's immune system. The nature of this intestinal colonization is initially determined by early exposure to environmental sources of microbes and by the general state of health of the infant. Whether the infant is breast-fed or formula-fed also has a strong influence on the intestinal bacterial population.
Human milk contains a number of factors that may contribute to the growth and population of the gut microflora of infants. Among these factors is a complex mixture of more than 130 different oligosaccharides that reach levels as high as 8-12 g/L in transitional and mature milk. Kunz, et al., Oligosaccharides in Human Milk: Structure, Functional, and Metabolic Aspects, Ann. Rev. Nutr. 20: 699-722 (2000). These oligosaccharides are resistant to enzymatic digestion in the upper gastrointestinal tract and reach the colon intact, where they then serve as substrates for colonic fermentation.
Cow's milk and many commercially available infant formulas that are based on cow's milk provide only trace amounts of oligosaccharides; as a result, prebiotics may be used to supplement the diet of formula-fed infants. Prebiotics have been defined as non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of cells in the gastrointestinal (GI) tract that can improve the health of the host.
Both the interaction among dietary components and among the microflora of the intestinal ecosystem are very complex. Consequently, then, the matrix of an infant formula or other pediatric nutritional composition may influence the effectiveness of prebiotics when such ingredients are provided as supplements in the diet of a formula-fed infant. Further, the type and concentration of lipids and proteins used in a formula matrix may also modulate the intestinal microbiota. Because human milk is the preferred source of infant nutrition, it is desirable to provide a formula matrix that mimics the qualities of human milk by allowing for effective supplementation of prebiotics as functional food ingredients.
Recently, it has been found that combining polar lipids with prebiotics, especially polydextrose (PDX) and galacto-oligosaccharides (GOS), lactoferrin, and long-chain polyunsaturated fatty acids (LCPUFAs) can lead to certain unique benefits. Accordingly, it would be beneficial to provide a nutritional composition for pediatric subjects that contains such a combination.